Inter-shaft bearings are located between two rotary shafts: constituted by two bushes interdependent with each of the shafts and with runners, or rollers, placed between them, the inter-shaft bearings allow each of the shafts to rotate independently while minimising friction and vibrations. In particular, in the context of an aeroplane turbine engine, this type of bearing can be found between the high-pressure turbine and the medium pressure turbine, mounted on concentric shafts that rotate in the same direction or in the reverse direction. Lubrication is then provided by an oil of aeronautic quality which circulates in the bearing along preset paths. An example is given in the document GB-A-2 358 678.
To this end, as shown by FIG. 1, a lubricant is injected onto the roller 1 at the level of the inner bush 2. A run path A, B is specified: by means in particular of the centrifugal force, the oil circulates along the roller 1, reaches the inner surface of the outer bush 3, and is drained along the inner surface towards the run paths A, B. In the context of FIG. 1, one part circulates upstream A and one part in the downstream direction B, on each side of the runners 1.
In normal running operations, the rolling bodies 1, 2, 3 are separated by a film of oil the thickness of which depends on the operating conditions of the bearing (such as the contact pressure, the relative speed of the rolling bodies, the piezoviscosity characteristics of the lubricant) and which allows surface deterioration arising out of the relative movement, from overheating for example, to be reduced. Usually, the input of the contact between outer bush and roller is considered as sufficiently supplied (indeed, the thickness of the surface film is often more than 20 times greater than that of the hydrodynamic film at the contact centre); however, when the thickness of the surface film is substantially reduced, the contact is no longer sufficiently supplied with lubricant and the result of this is a reduction in the height of the hydrodynamic film; then occurs a phenomenon of insufficient supply. Two main root causes can be identified for such an insufficiency of supply:
In high-speed running, and principally where rolling bearings are involved, the frequency with which the rolling bodies run on the roll track means that the surface film does not recover its nominal value between two runs of the rollers. At high-frequency, the result is a substantial reduction in the height of the hydrodynamic film.
If for a stationary bush, the thickness of the surface film is mainly a function of the surface tension characteristics of the fluid, when the outer bush is in rotation, the thickness of the surface film also additionally becomes a function of the centrifugal field, the fluid characteristics and the distance from the point under consideration relative to the drainage orifices (located on either side of the roll track).
Inter-shaft roller runs, particularly when the shafts are counter-rotational, are therefore an area favourable to the appearance of phenomena of insufficient supply, particularly the bearings used in aeronautics in the light of the rotation speeds involved.
Furthermore, a proper dimensioning of a high-speed run consists in providing an oil feedback circuit that is sufficiently permeable for the oil not to be able to accumulate on the track, the consequence of which would be an unacceptable increase in dissipated power.
Compensating for an insufficient supply of lubricant in respect of an inter-shaft bearing is therefore an issue if a jet turbine engine for example is to operate properly.